Mass Spectrometer Question #2

1. The problem statement, all variables and given/known data
A singly charged positive ion has a mass of 2.50 x 10^-26kg. After being accelerated through a potential difference of 250 V, the ion enters a magnetic field of 0.500 T, in a direction perpendicular to the field.

2. Relevant equations
Calculate the radius of the path of the ion in the field.

3. The attempt at a solution
Firstly, I drew this picture to try to help me with the problem:
http://img411.imageshack.us/img411/4197/picxc7.jpg [Broken]

I've tried to set Fm = Fe, therefore coming up with the derived equation of V=E/B, but I don't know E.

I then tried to find the electric field by using the voltage, but I don't know the distance. (V=ED)

I also tried setting Fm = Fc, getting Bqv = mv^2/r, and finally resulting in Bqr/v=m. I had B, q, and m, but I didn't have r and v.